Reflow soldering is employed extensively in the surface mount industries and particularly in the automated manufacture of printed circuit boards. Generally, miniature electronic components are surface mounted on a printed circuit board to which a solder in a creamy or paste-like consistency has been applied by a method such as screen printing, stenciling or dispensing.
The printed circuit board is then subjected to a sufficiently high temperature, generally 50.degree. C. greater than the melting point of the alloy, to cause the flux and the alloy in the solder to liquify and to contact the components so that upon subsequent cooling of the printed circuit board, the components are soldered in place on the board. The heat can be supplied by, for example, infrared, vapor phase, heated conveyor belt (hot belt) or convective means.
The solder is conventionally comprised of a soft powdered metal alloy dispersed in a liquid medium containing a flux, an organic solvent, and a thickening agent specially selected to impart the desired consistency to the mixture. Ideally, the flux component should be non-corrosive, thereby yielding flux residues after completion of soldering which are themselves non-corrosive and non-conducting. In practice, however, such is not the case. Rosin-based flux materials, such as abietic acid-based flux, are used in most commercially available solders specifically formulated for use in the surface mount industries. These fluxes commonly contain activators, such as halides, particularly bromides, which leave corrosive and conductive residues requiring expensive and time-consuming removal techniques. Conventionally, these removal techniques utilize organic solvents, e.g. fluorochlorocarbons which give rise to environmental problems. As an alternative, aqueous systems have been tried for residue removal. However, due to poor wetting, it is difficult to obtain the penetration necessary with such systems to achieve the required cleaning. Additionally, removal of flux residues is difficult, particularly from areas of printed circuit boards beneath the components soldered thereto.
Rosin-based fluxes have additional disadvantages whether or not they contain conventional activators. For example, corrosive, rosin-based flux residues tend to be sticky, thereby inhibiting the automatic testing of the circuits and proving aesthetically objectionable.
The use of rosin-based or mildly activated rosin-based flux-containing solders can also result in poor wetting by the solder of the substrate and of components to be soldered.
Flux residues tend to be hygroscopic, thereby causing spattering, and some fluxes also undermine solder joint integrity by mechanisms such as permitting alloy particles in the solder to migrate from the solder site, giving rise to the formation of numerous small discrete balls of soft solder around the soldered joint, in effect creating electrical short circuits.
Hedges, et al, U.K. Patent No. GB 2,198,676 have attempted to solve the flux residue problems with a solder formulated without the rosin-based flux whereby the liquid medium in which the powdered alloy is dispersed comprises a substantially water-immiscible organic solvent, such as terpineol, containing at least one organic acid other than a rosin or a modified rosin, an amine or an amine hydrohalide as a flux; and at least one thickening agent. This non-rosin-based flux containing formula is commercially available as Multicore.RTM. X-32 from Multicore Solders, Hertfordshire, England.
However, the Hedges, et al solder still yields a discernable residue when reflowed in air even when the recommended temperature profile for heating is followed. It also wets poorly. This suggests that the operation window for reflow operations in air with the above solder is narrow and consequently difficult to practice commercially.
Oxidation on the surface of molten solder in lead tinning processes has been controlled by utilizing a nitrogen-purge system eliminating any contact of oxygen with the solder. See The Welding Journal, Vol. 65, No. 10, p 65 (1986). Lead tinning is performed on components prior to any soldering operations. Nitrogen has also been demonstrated to reduce white haze and to increase the chances of soldering marginally-solderable components. It is also suspected of reducing nonwetting, opens, solder balls, bridges and misalignments. See M. J. Mead and M. Nowotarski, The Effects of Nitrogen for IR Reflow Soldering, Technical Paper, SMT-IV-34, presented at the SMART IV Conference, Jan. 11-14, 1988.
The present invention surprisingly overcomes the above-mentioned problems and ensures good wetting of the substrate and of the components. In addition, the present invention leaves only a very mimimal post-solder residue and thereby eliminates post-solder cleaning operations. It is therefore an object of the invention to provide a method of reflow soldering which minimizes post-solder residue. It is a further object of the invention to provide a method of reflow soldering that can operate over a broad range of temperatures. It is yet a further object of the invention to provide a method of reflow soldering which ensures good wetting of the substrate and of the components. Another object of the invention is to provide a method of reflow soldering which exposes the components to maximum temperatures for a relatively short period of time.